LT3437IFE#TRPBF Linear Technology, LT3437IFE#TRPBF Datasheet - Page 12

• Manufacturer Part Number: LT3437IFE#TRPBF
• Description: IC REG SW HV 500MA 16-TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3437IFE#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 54 V
• Current - Output: 500mA
• Frequency - Switching: 200kHz
• Voltage - Input: 3.3 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3437
APPLICATIO S I FOR ATIO
high input current surge during start-up. The soft-start
function reduces input current surge by regulating switch
current via the V
rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from
the C
dV/dt. When the feedback voltage is below 0.4V, the V
will rise, resulting in an increase in switch current and
output voltage. If the dV/dt of the output causes the current
through the C
reduced resulting in a constant dV/dt at the output. As the
feedback voltage increases, I
increased dV/dt until the soft-start function is defeated
with 0.9V present at the FB pin. The soft-start function
does not affect operation during normal load conditions.
However, if a momentary short (brown out condition) is
present at the output which causes the FB voltage to drop
below 0.9V, the soft-start circuitry will become active.
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the
voltage ripple this causes at the input of LT3437 and
force the switching current into a tight local loop,
thereby minimizing EMI. The RMS ripple current can be
calculated from:
Ceramic capacitors are ideal for input bypassing. At 200kHz
switching frequency input capacitor values in the range of
2.2µF to 10µF are suitable for most applications. If opera-
tion is required close to the minimum input required by the
LT3437, a larger value may be required. This is to prevent
excessive ripple causing dips below the minimum operat-
ing voltage resulting in erratic operation.
Input voltage transients caused by input voltage steps, or
by hot plugging the LT3437 to a pre-powered source such
as a wall adapter, can exceed maximum V
sudden application of input voltage will cause a large
12
I
RIPPLE RMS
SS
pin to the output determines the maximum output
(
SS
)
=
C
capacitor to exceed I
pin to maintain a constant voltage ramp
I
OUT
V
U
IN
V
U
OUT
CSS
(
V
increases, resulting in an
IN
–
W
CSS
V
OUT
, the V
)
IN
ratings. The
C
U
voltage is
C
pin
surge of current in the input leads that will store energy in
the parasitic inductance of the leads. This energy will
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of the input capacitor and LT3437. All input voltage
transient sequences should be observed at the V
the LT3437 to ensure that absolute maximum voltage
ratings are not violated.
The easiest way to suppress input voltage transients is to
add a small aluminum electrolytic capacitor in parallel with
the low ESR input capacitor. The selected capacitor needs
to have the right amount of ESR to critically damp the
resonant circuit formed by the input lead inductance and
the input capacitor. The typical values of ESR will fall in the
range of 0.5Ω to 2Ω and capacitance will fall in the range
of 5µF to 50µF.
If tantalum capacitors are used, values in the 22µF to
470µF range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2:1 for high surge applications.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR) because this is what deter-
mines output ripple voltage. To get low ESR takes
volume, so physically smaller capacitors have higher
ESR. The ESR range for typical LT3437 applications is
0.05Ω to 0.2Ω. A typical output capacitor is an AVX
type TPS, 100µF at 10V, with a guaranteed ESR less
than 0.1Ω. This is a “D” size surface mount solid
tantalum capacitor. TPS capacitors are specially con-
structed and tested for low ESR, so they give the lowest
ESR for a given volume. The value in microfarads is not
particularly critical, and values from 22µF to greater
than 500µF work well, but you cannot cheat Mother
Nature on ESR. If you find a tiny 22µF solid tantalum
capacitor, it will have high ESR and output ripple
IN
pin of
3437fc